Topic: 1973 cb500 stock carb settings

[miller_87]

Okay, here's my problem, last year I optained a 73' cb500 motorcycle for free, and rebuilt the bike.  The bike has strait pipes on, but I noticed when rebuilding the carbs that it was never re-jetted because I was putting the same jets in from the kit as were already in the carb.  I ran it last summer, and now have decided to put a 4-1 MAC exhaust system in for more back compression.  When running last summer, it had really poor takeoff when first putting it in gear, and really did not wanna run below 3,000 rpms in gear.  My guess is that all they did when putting the new exhaust on was adjust the clip on the main needle in the carbs.  My question is, does anybody know the stock position of the clip on the main needle?  My guess is this is all the changed.  The plugs never seemed to be running lean, but if that main needle is set way up, then the plugs will appear rich, and that may account for my poor gas mileage I was getting at 32-35mpg on a cb500    So if anybody has any useful information for me on the stock carb settings on the main needle or any tips otherwise...let me know....THANKS TO ALL

[n9viw]

Well, I had a banger of a reply all whipped up, and Firefox locked up on me. Oh well, computers is all black magic anyway, I shouldn't even be playin with em!
Yes, your jetting was way off, thanks to those drags (BTW, how much might you want for em, if you're selling em?) Stock jetting was 38 or 40 primary, 100 main, clip in the middle notch of the needle, and idle screw 1.25 turns out, if it was anything like my 550. I bet you had weird transitions, too- sudden flat spots or surges of power- what with the stock idle and main jetting and WAY RICH needle setting!
Also, you may find that a 4-1 makes the stock settings a little RICH, if anything. Do a proper set of plug chops in each range (idle, 1/4, 1/2, 3/4, and full throttle), and change plugs between each one. I use Champion plugs to do my plug chops, because while they don't run well, they show burn color truer than NGK plugs do, and they're CHEAP.

[TwoTired]

Both the exhaust and air filtration changes can effect the jetting requirements for the carbs, because they effect how the engine breathes.
Further, air filter restriction acts as partial choke and effects the draw of fuel from the jets and needle metering orifices.  The stock jets allow for stock air filtration.
Assuming you are using the stock 627B carbs, the slows were #40 and the mains were #100.   The stock main needle was tapered 2.5 degrees.  But, I haven't found a spec for the stock needle clip position on the 500 carbs.  However, unless you are using the stock exhaust, it probably doesn't matter.  You will have to find a clip position that operates properly with your air filter and exhaust restriction characteristics.  Unless you have access to a Dynomometer, the plug chop method will likely be your best bet at correct carb mixture settings.  Begin with the main jet at WOT under load.  Then move on to midrange throttle settings to determine needle height.  I'm not certain there is enough heat at idle setting to get a proper mixture read on the plugs.  It might be better to tweak this with the throttle roll on technique. First, check your air bleed screws to see if they are still the stock hollow type.
Since your carbs do not have accelerator pumps, the idle settings are set on the rich side so throttle changes of 1/2 a twist result in smooth pickup.  So, keep turning your idle screws out about 1/8 turn at a time until the engine wheezes at low speed pickup with the 1/2 turn twist.  Then turn them back in 1/8 turn and go ride.

Carb tuning is the LAST adjustment to make to the bike AFTER all the tuneup items have been addressed, as well as all induction and exhaust components are selected.

Cheers,